This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-333194 filled on Oct. 30, 2001.
The present invention relates to an ignition coil, which can be directly installed in a plug hole of each cylinder of an engine.
An ignition coil generates a high voltage caused by the mutual induction for an ignition plug. For example, an ignition coil disclosed in U.S. Pat. No. 6,208,231 B1 (JP-A-11-111545) is shown in FIG. 7.
The ignition coil 100 shown in FIG. 7 is inserted in a plug hole of an engine. A case 117 has a cylindrical shape and forms an outside shell of the ignition coil 100. In the center of the case 117, a center core 101 that is like a pole is installed. Around the center core 101, a secondary cylindrical spool 102 is installed. A secondary coil 107 is wound around the secondary cylindrical spool 102. A primary cylindrical spool 113 is installed around the secondary coil 107. A primary coil 115 is wound around the primary spool 113. The inside of the case 117 is filled with an epoxy resin 110 to keep the insulation between the above members, and the epoxy resin 110 glues the members to each other.
However, the coefficient of linear expansion of the members stored in the case 117 is different from that of the epoxy resin 110. Therefore, the members exert a thermal stress to each other by the compressibility or the expansion of the members caused by transition of a temperature. For example, the inner members in the case 2 such as the secondary spool 102 and the central core 101 are especially influenced by the thermal stress.
Therefore, in the ignition coil 100, the central core 101 is covered with a stress relaxation member 105. The stress relaxation member 105 is made of a lubber material and possesses the elasticity. By the elasticity, the stress relaxation member 105 can deform, and the thermal stress between the central core 101 and the epoxy resin 110 is relieved.
In the ignition coil 100, the primary spool 113 is covered with a thin layer of a film 114. The film 114 is made of a PET material, so that the adhesion to the epoxy resin 110 is weak. Therefore, the primary spool 113 and the primary coil 115 can be separated easily. By the separation, the thermal stress from the primary coil 115 to the primary spool 113, that is from the outer side to the inner side, is cut off.
Consequently, the diameter of the coil 100 is large. Because, the stress relaxation member 105 should be placed around the central core 101, and the thin layer of a film should be covered around the primary spool 113. That is, the diameter of the ignition coil 100 is limited by the diameter of the plug hole in its design.
Furthermore, the number of the members is large because the stress relaxation member 105 and the thin layer film 114 are placed. Therefore, the manufacturing cost is high. The manufacturing process is complex because the process to cover the central core 101 with a thin layer of a film is necessary.
It is an object of the present invention to provide an improved ignition coil.
According to the present invention, an ignition coil is formed to have an air layer as an insulator instead of an insulated resin in a gap between a central core and an inner spool or a gap between an outer spool and a case.